The present invention relates to an ultrasonic imaging apparatus and, more particularly, to an ultrasonic imaging apparatus having a color display function.
Ultrasonic imaging apparatuses are used in various fields, particularly in medicine. Ultrasonic imaging apparatuses for displaying a blood flow have been developed. These apparatuses form an image of a blood flow and can display the image in color. The image, which is a motion picture, shows the direction and flow velocity of blood flow in an object to be examined. The apparatuses, however, have functional limits, and display the image of blood flowing at a velocity exceeding a predetermined value in a color different from that used for displaying the actual blood flow velocity. In order to eliminate this drawback, an ultrasonic imaging apparatus adopting a Doppler method has been developed, which displays the result of frequency analysis at a certain point by using FFT (Fast Fourier Transform). This imaging apparatus has a funcion called zero shift or base-line shift. According to this function, when a measurement value exceeds a display limit, it is shifted downward. For example, when a blood flow velocity which changes over time exceeds the upper limit (+MAX) of the measurement range, the exceeding portion is displayed on the lower section of the display range (upper limit +MAX to lower limit -MAX), i.e., a phenomenon called aliasing occurs. Then, an operator may misunderstand the displayed image and take it for a reverse blood flow because of aliasing. In order to prevent this, the zero line of the axis of ordinate is subjected to a zero shift in order to shift the display range in the reverse flow direction. As a result, the maximum value of the flow velocity is less than the upper limit (+MAX).
In the ultrasonic imaging apparatuses, the address of a RAM corresponds to the flow velocity. When the zero shift is performed in the conventional apparatuses, data is written at an address shifted by a zero shift amount, or is read out from a memory area at an address shifted by the zero shift amount. Since a Doppler signal is sampled in synchronism with an ultrasonic rate pulse, an upper limit exists in the measurable flow velocity range, and aliasing occurs in a flow velocity component exceeding the upper limit. Aliasing in this case is expressed as a change in color of the displayed image. More specifically, assuming that forward and reverse flows are displayed in red and blue, respectively, a flow velocity component, which should originally be displayed in red and exceeds the upper limit, is displayed in blue because of aliasing. Although the operator may not mistake the aliasing portion displayed in blue as a reverse flow, it is preferably displayed in red.
According to the conventional Doppler method, the blood flow direction cannot be discriminated, unlike in blood flow imaging. Therefore, ultrasonic beams are incident from various directions, thereby finding a portion at which a maximum flow velocity can be obtained. With this method, however, since the incident directions of the ultrasonic beams cannot be arbitrarily set because of the shape of an ultrasonic probe or the positional relationship among internal organs of the patient, an error often occurs in the maximum flow velocity, depending on the ultrasonic beam incident angle. A portion at which the maximum flow velocity can be obtained must be measured while the sample volume position is adjusted, resulting in a very cumbersome operation.